Pigments and paints and method of making same



Patented Dec. 18, 1945 PIGMENTS AND PAINTS AND METHOD OF MAKING SAMELords E. Barton, Windsor, Conn.

No Drawing. Application November 5, 1942, Serial No. 464,659

7 Claims. (01. 106-256) This invention relates to lead, silicatepigments and paints and. more particularly to a pigmentary lead silicatewhich is especially adapted for use as a tint base when ground withlinseed oil or other suitable vehicle.

This application is a continuation-in-part of my United Statesapplication Serial No. 359,769,

filed October 4, 1940, and includes ,a fuller disclosure of the methodof making paint products with lead silicate pigments wherebyexceptionally durable exterior paints can be prepared.

Such fuller disclosures are based upon actual exposure tests resultingfrom longer and additional experience in the use of lead silicatepigments since filing the prior appllcation referred I have discoveredthat useful opaque lead silicate pigments can be made by heating amixture of lead oxide-or a lead compound which upon heating yields leadoxide, and silica (silicon dioxide, SiOz) under controlled temperatureconditions whereby the reaction to form lead silicate'takes place in thesolid phase without fusion,

thus ensuring a soft fine texture and other desirable physical qualitiesin the pigment product. Furthermore, I have discovered that paintproducts prepared with lead silicate pigments and a suitable vehicle,such as linseed oil with or without modifying addition, have exceptionaldurability under exterior exposure conditions and are substantially freefrom the usual defects of exterior paints for unusually long periods.Such improvement of exterior paint" products, now verified by longexposure tests, are of far reaching importance, for the paints not onlyprovide 7 far more satisfactory and economic protection of both wood andmetal surfaces, but even more importantly, particularly under thepresent conditions of war economy, wouldresult in the saving of a largeamount of lead since a unit weight and volume of lead silicate pigmentcan be prepared with a greatly reduced lead content as compared withother types of lead pigments.

By my process not only can I make lead silicate pigments of the regularmolecular proportions such, for example, as subnormal lead silicate,PbSlzOs, normal lead silicate, PbSiOz, and basic lead silicate, Pb2SiO4,but I can make also equally mal lead silicate, a product having anequimolec-' ular ratio of PbO to SiOz, by subnormal lead silicate, a,product having a ratio of PhD to SiOz less than equimolecular and bybasic lead silicate a product having a ratio of PbO to SlOz greater thanequimolecular. Lead silicate pigments other than the normal PbSiOs, maycontain, and some of them probably do contain, more than one definitechemical compound, the various silicates, basic, 'normal and subnormal,adjusting during the heating process proportionally in the product inaccordance with natural laws to the state of most stable equilibrium. Ican also make useful lead silicate pigments containing a higherpercentage of silica than the regular formula, PbSizOs, but, thoughsuchla pigment is cheaper, its hiding power is reduced and thus there isa practical limit based upon utility of the product in its application.Such a practical limit appears to be a subnormal lead silicatecontaining not less than 85% actual lead silicate calculated as regu- Ilar subnormal lead silicate, PbSizOs, and about 15% silica. The excesssilica may be chemically or phsically combined or, possibly, partiallyin the free state.

Before giving detail examples showing the op eration of my process andcharacteristics of the various products I shall give some generalinformation relative to the raw materials, process and product.

The materials used should be as free from impurities as possibleconsistent with industrial availability, since small amounts of iron orother impurities tend-to discolor the product. Either litharge (P100),or other oxides of lead such as red lead (Pb304) or lead peroxide(PbOz), may be used as a source of lead. Other lead compounds whichyield the oxide upon heating may also be employed, such as white lead(2PbCOc.PbH2O2)-. A pure grade of finely divided silica, either naturalor artificially prepared, is suitable,,in either hydrous or anhydrousform..

I first thoroughly mix the materials in the required proportions, eitherby dry-milling or, preferably, by wet-milling, thereby reducing particlesize and also assuring a very close association of the ingredients. Themilling may be done either in a batch or a continuous type of mill; ineither case'by theme of a porcelain or silex lining to avoidcontamination of materials.

The slurry from the wet-mill is dewatered as by settling anddecantation' and/or filtration. Any suitable equipment may be used, suchas D9111 tanks and filter presses.

The wet pulp is then charged directly into the heating furnace, or itmay be preliminarily dried and then charged into the furnace. Thefurnace may be either of the batch type such as a muiiie, hearth furnaceor Bruckner furnace, or of the continuous kiln type. In either case alining of high-grade fire brick is satisfactory to avoid contaminationof materials. Whatever the type of furnace employed;-means should beprovided for control of temperature: In furnaces where products ofcombustion contact the charge,

a neutral or oxidizing flame should be maintained to avoid possiblereduction of metallic oxides.

The time required in the calcining z'one de-- pends upon thetemperature. The higher the temperature within the permissible range,the shorter the time; however, there is a tendency to clearer colortones at the lower and medium temperatures within that range. Lead oxideand silica react at a. temperature of 500 C. and at 550 C. I can make asubstantially white lead silicate pigment by heating for a period of. 48hours. per limits of temperature range 650 C. for the subnormal, 680 'C.for the normal and.725 C. for the basic lead silicate are exceeded for aconsiderable length of time.

While all my lead silicates can be made with in a temperature range of550 to 625 C., I prefer the following temperature ranges since at suchtemperatures the reaction is fairly rapid and yields a product having asoft texture and good hiding power; for lead silicates containing about55 to 70% PbO, 5'75 to 610 C., for lead silicates containing about 70 to85% PhD, 600 to 650 C.; for lead silicates containing above 85%PbO,/-6'40 to 680 C.

Within the preferred temperature ranges specified the silicates require1 to 2 hours in the calcining zone of the furnace. While the reaction isusually complete in 1 to 1% hours, it is desirable to hold the charge atthe proper temperature a little longer than appears absolutely necessaryto insure that the reaction is complete. progress of the reaction duringcalcination may be judged by withdrawing a sample from the furnace forinspection. The finished product should be substantially white andshould not acquire a yellower color upon milling'or grindin the samplein a mortar.

Lead silicate pigments. if not calcined at too high a temperature, aresoft in texture and fine enough to be used as pigments for paint, butusually milling is required to break down aggregates, reduce particlesize and develop full tinting strength and hiding power of the products.

Either dry or wet-milling methods may be used with or without airseparation or wet classification respectively, thereby assuringuniformity of product. If wet-milling has been employed, the slurry isdewatered, dried, and if necessary passed through a disintegrator tobreak downlumps and aggregates.

The following examples show the working of my pigment process andproperties of the resulting lead silicate pigments, but such examplesshould not be construed as limiting the application of my process sincemany other lead silicate pigments can be made within the general limitsof proportions and temperatures.

Example 1.-Su,bn0rmal lead silicate, PbSizOs slurry dewatered and dried.The mixture was then calcined for 2 hours at 590 to 610 C. The

product after milling had the following properties:

on absorption 10.05 lbs. per 100 lbs. of pigment. Hiding power 70compared with white le d standard taken as 100 Example 2.--N01'mal leadsilicate, PbSiOa 158 parts of lead oxide (litharge, PbO) and .42 partsof silica were wet-milled together and the slurry dewatered and dried.The mixture I was. then calcined for 2 hours at 610 to 625 C.

Lead silicates sinter and fuse if the up- Chemical composition:

Lead oxide per cent.-- 78.80 Silica do 21.20

Oil absorption 7.37 lbs. per 100 lbs. of pigment. Hiding power 85compared with white lead standard taken as 100 Example 3.-Basic leadsilicate, Pb2SiO4 1'76 parts of lead oxide (litharge, PbO) and 24 partsof silica were wet-milled together and the slurry dewatered and dried.The mixture was then calcined for 2 hours at 665130 675 C. The

product after milling had the following propere Specific gravity 7.03

Chemical composition:

Lead oxide ..l. per cent 88.2 Silica do....- 11.8

' on absorption 6.55 lbs. per 1. oo lbs. of pi The ment. Hiding powercompared with white lead standard taken as Example 4.-Normal leadsz'lzdate, PbSiOs 162 parts of basic lead carbonate (2PbCO3-PbH202) and38 parts of silica lwere wet-milled together and the slurry dewateredand dried; the mixture was then calcined for 1 /2 hours at 610 to 630 C.The product after milling had the following properties:

Specific gravity 5.86 Chemical composition:

Lead oxide per cent 78.80 Silica do 21.20

Composition of pigment:

Lead oxide, PbO "percent" 85.00 Silica do 15.00

Physical properties:

Specific gravity 7.01 Oil absorption (parts of oil perj100 parts ofpigment) 7.00 Tinting strength (compared with white lead standard=l00)100 Analysis of pigment Lead oxide, PbO percent 55 Silica do 45 Physicalproperties: 2 Specific gravity 4.56

Tinting strength (compared with white lead standard=l) 70 Though thelead silicate pigments made as described are substantially white or verylight colored when freshly prepared they are not invariably stable tothe action of light and may acquire a gray or gray-brown tone or colorin a few hours under the action of direct sunlight or after a longerperiod in diffused light. .All of the lead silicate'pigments whethersubnormal,

normal or basic are to some extent unstable to light but the instabilityincreases with the basicity or content of lead oxide in the pigment. Ihave also found that the unstable characteristic of the pigment stillpersists to some extent after grinding with drying oil vehicles such aslinseed oil, tung oil, and oil varnishes. The resulting paints uponexposure to light may acquire a grayish color which gradually becomesdarker. p

I have discovered that notwithstanding 'the darkening or graying effectwhich results upon exposure to light, my pigmentary lead silicatepigments made by the process indicated when compounded with a suitablevehicle, such as linseed oil or other suitable vehicle, yield paintshaving exceptional durability when exposed to sunlight and weather beinghighly resistant to both checking and chalking. While all my leadsilicate pigments, subnormal, normal and basic, impart exceptionaldurability, I have observed from long paint-exposure tests that thebasic lead silicate paints are appreciably more durable than those madewith the subnormal and normal pigments. The general durability impartedto paints by lead silicate pigments adapts them to a particular field ofapplication which will be further considered in this specification.

Two classes of white pigments are generally recognized: (1) The opaquewhite pigments, such as white lead, zinc oxide and titanium pigmentswhich impart opacity and whitness when mixed with oil or other organicvehicles; and (2) The extender pigments, such as whitingand china claywhich do not impart appreciable opacity when similarly mixed withvehicles.

The opaque pigments of class 1 are further subdivided into two classes:(0) Those which are reactive or capable of combining chemically orphysico-chemically with ingredients of the vehicle, such as white leadand zinc oxide and (b) Those which are chemically inert, such astitanium pigments.

My new lead silicate pigments belong to the class of opaque reactivepigments, though they difier in degree of reactivity, i. e., thecapacity to combine chemically or physico-chemically with the oleaginousconstituents of film-forming vehicles in which they are incorporated, asindicated by the rate of drying when the pigments are mixed'with asuitable vehicle and exposed 'as a paint film. The rates of drying ordegrees of reactivity. are in the order of the lead oxide contents ofthe pigments.

In the field of paints for exterior use much investigation has beenconducted in an effort to combine the special features of the variouswhite pigments and vehicles to overcome the defects in paint filmsresulting from exposure to light and weather; and cracking, scaling,fading of color and dirt collection. The general durability of exteriorpaints is directly related to its resistance to chalking, checking,cracking and scalingbut it is necessary to consider also the appearanceor decorative value of the paint which is largely'dependent upon suchfactors as gloss retention, tint retention or non-fading of color andcleanness of surface "or resistance to dirt collection. Checking,cracking and scaling can be partially controlled by regulation ofpigment volume concentration in the various coats of paint but thedefeet is of frequent occurrence with most combinations of commercialwhite pigments and. linseed oil vehicle and is inevitable with some ofthe white pigments so compounded.

Deterioration of the paint film by chalking of white paints, if notexcessive, may be advantageous since the washing away of the chalkformed carries withit much of the adherent dirt collected during earlierexposure, thus presenting a cleaner surface. The deterioration of tintedThe outstanding characteristic of the lead silicate pigments is theircapacity to impart durability to paints applied to either wood or metalsurfaces. From the standpoint of both economy and durability leadsilicates made by the process described are especially adapted as a basefor tinted and colored paints. The pigment in tinted or colored paintsis largely white, usually including some opaque white pigment, withlesser quantities of colored pigments. The colored or tinting pigmentoften supplies all the, obscuring power or opacity required therefore.the white base or tint base should not have greater opacity or hidingpowerthan necessary since more of the colored pigment would then berequired thus in most cases increasing the cost. As before shown bydetail examples the opacity or hiding power of my lead silicatepigments, while ample to afford good hiding power in paints, is notexceptionally high and they are for that reason particularly suitablefor use as a tint base.

As before stated, the degree of instability or darkeningof lead silicatepigments appears to be proportional to their lead content and thereforecates are best adapted for making such tints as ing effect under theaction of light will be obsoured by the tint or color desired.

In using my lead silicate pigments as a tint base I have discovered thatcompounding them Such defects are chalking, checking Pigmentcomposition, weight percent with the coloring pigment and a vehicleconsistin substantially of linseed oil gives particularly satisfactoryresults as regards general durability, resistance to chalking and fadingand retention of gloss; The linseed oil vehicle of course includes thenecessary small amounts of thinner,

' The use of to of heavy-bodi d linseed oil with the balance raw orrefined linseed oil permits the application of a thick priming coat ofhigh pigment volume concentration which can be followed by a finishingcoat thus completing the painting in two coats, I have found itpreferable to impart maximum durability to the paint by using only thelead silicate and the color as pigments for the paint but other pigmentsmay be blended with the lead silicate with very satisfactory results ifdesired.

If it is desired to use lead silicate pigments for j making white paintsit is preferable to use the pigments which have been stabilized againstdiscoloration by light as disclosed in my United States Patents Numbers2,233,042 and 2,236,051 and in my copending United States patentapplication Serial Number 428,035 filed January 24,

The following examples in :Iables I and II show the application of mylead silicate pigments in the preparation of paints for exterior use.Table I shows the paint formulation and Table II shows the condition ofthe paint films after exposure.

The paints were prepared by grinding the pigments with enoughof the oilto form a smooth paste and then reduced with the remainder of the oil,thinner and drier to proper consistency for painting.

The painted panels were all exposed in a vertical position facing south.

It is apparent from Examples 1, 2 and 3, in the tables, that after anexterior exposure for a period of 4 to 5 years the paint films are soundand apparently would afford ample protection for a period of severalyears longer. This result is exceptional in view of general experiencewith exterior paints which has been that many paints have entirelybroken down after four years exposure and in the majority of cases thestructure needs repainting.

The substantial absence of fading and chalking in the gray paint ofExample 1 indicates the special adaptability of lead silicate pigmentsin the preparation of tinted paints for exterior use.

The paints of Examples 4 and 5 have been formulated to adapt them .fortwo-coat exterior painting. The clean surfaces and absence of fading andother defects after two years exposure indicate the advantages of tintedlead sillcate paints of this type for exterior use.

Table I.-Paint formulation Formula number Normal lead silicate+0.5%lampblack.

Paint composition, percent by weight:

igment White Basic lead silicate,

Normal lead silicate+2.0% Indian red.

Basiclead silicate,

Table II .Results of exposure-Condition of paint Formula number Timeexposed, months 51 G ss Checking and cracking. N Chalking FadingDarkening by exposure to light.

Dirt collection General condition Very little ExcellenL- None.Excellent.

None Excellent Very little Good 1 Very slight alllgatoring in surfacecoat only-no rusting.

1942, but lead silicate made. by the process of While I haveparticularly described the applithe instant case can be used by blendingit with cation of my novel lead silicate pigments in the highly opaquepigments such as titanium pigments in proportions to substantiallyobscure the darkening of the lead silicate when exposed to light. Such acombination results in a white paint of improved opacity and durability.

preparation of paints with oleaginous vehicles, I do not limit myself tothat type of vehicle for'I have found these lead silicate pigments ofgeneral adaptability. In general the properties of my lead silicatepigments adapt them for application to oil, enamel and lacquer type pints and for other purposes where white pigments are used,.

such as for rubber, linoleumand other floorcoverin'gs, printing inks,vitrified enamel, etc.

I claim as my invention:

1. As a new composition of matter a protective coating compositioncomprising an oleaginous, film-forming and drying vehicle pigmented withan essentially pure anhydrous opaque white lead silicate pigmentphysico-chemically reactive with said vehicle, said pigment being formedby heatlead silicate pigment physico-chemically reactive I ing a mixtureof lead compounds which upon heating will supply to the product onlylead oxide in an amount between about 40% and about 88% (PbO) and silicaat a temperature suificiently high to chemically combine the oxides inthe solid phase without fusion.

2. As a new composition of. matter a protective coating compositioncomprising an oleaginous, film-forming and drying vehicle pigmented withan essentially pure anhydrous opaque white normal lead silicate pigmentphysico-chemically reacted with said vehicle, said pigment being formedby heating a mixture of substantially equimolecular proportions of leadoxide and silica at a temperature sufllciently high to chemicallycombine the oxides in th solid phase without fusion.

3. As a new composition of matter a protective coating compositioncomprising an oleaginous, film-forming and drying vehicle pigmented withan essentially pure anhydrous opaque white subnormal lead silicatepigment physico-chemically reactive with said vehicle,-said pigmentbeing formed by heating a mixture of lead oxid and with said vehicle,said pigment being formed by heating a mixture of lead oxide and silicacontaining between about 88% lead oxide (PhD) at a temperaturesufliciently high to chemically combine the oxides in the solid phasewithout fusion.

5. As a new composition of matter a protective coating compositioncomprising an oleaginous, film-forming and dryingvehicle pigmented withan essentially pure anhydrous opaque white subnormal lead silicatepigment containing 40 to 78% lead oxid and the balance silicaphysicochemically reactive with said vehicle, said pigment being formedby heating a mixture of lead oxide and silica at a temperaturesumciently high to chemically combine the oxides in the solid phasewithout fusion.

6. As a new composition of matter a protective coating compositioncomprising an oleaginous, film-forming and drying vehicle pigmented withan essentially pure anhydrous opaque white basic lead silicate pigmentcontaining '79 to 88% lead oxide and the balance silicaphysico-chemically.

hydrous opaqu white lead silicate pigment physlco-chemically reactivewith said vehicle,

said pigment being formed by heating a mixture 4 ically combine theoxides in the solid phase without fusion.

LOUIS BARTON.

